A method of this type is known and comprises gluing each sheet to the strip edges opposite. The sheets provided with a relief or the edges of the strip material are provided with glue, after which the assembly comprising said sheets with the strip material between them is compressed. During this operation the relief in the sheets forms corresponding depressions in the strip material.
This known method has a number of disadvantages. Firstly, in practice it is not found to be simple to hold the sheets in the correct position with respect to the strip material during compression of the assembly. Initially only the highest parts of the relief of the sheets rests against the strip material, which leads to the sheets and the strip material shifting relative to one another parallel to their surfaces during the subsequent compression of the assembly. As a consequence the assembly acquires an uneven peripheral edge, one sheet protruding outwards further than the other.
For this reason it is necessary to use separate clamping elements which hold the various components of the assembly in the correct position with respect to one another during compression. The installation required for carrying out a method of this type is consequently relatively complex and expensive.
A further disadvantage of said known method is that the strip material of the core springs back to some extent. The consequence of this is that the thickness of the panel, comprising a core with sheets, having a relief, on either side, is greater before compression and the thickness finally obtained after gluing is smaller. This is highly undesirable in the case of the so-called cold-stack method, where a few tens of panels are stacked on top of one another. A stack of this type would result in too large a spring travel, with the consequence that the stack becomes unmanageable.
The aim of the invention is, therefore, to provide a method with which accurate production of such panels is nevertheless possible in a simple manner. This aim is achieved in that the core is compressed in order to pre-form depressions in which the relief of the sheet opposite can be accommodated and in that said sheet bearing a relief is then glued at least to those parts of the core not provided with depressions.
As, with the method according to the invention, the depressions are made in the correct position beforehand, the sheets can be placed with the associated relief in the correct position on the pre-formed core without any difficulty. The depressions in the core and the corresponding protuberances on the sheets ensure that the correct mutual positioning is guaranteed, even at the peripheral edges.
An additional advantage of making the depressions in the core beforehand lies in the strengthening effect of the parts of the core which are highly compressed in these locations. Said compressed parts give the core, which initially is fairly flexible, a certain rigidity in its plane, which facilitates treatment, for example during the application of glue.
The method can, in particular, be carried out using a core composed of material containing moisture-absorbent fibres. In that case the method comprises the successive steps of compression and then drying of the core.
With this method the core can, after compression, also be fed through the nip between two rollers in such a way that the strip edges are moistened, crushed immediately thereafter and then dried. The advantage of such crushed strip edges is that these provide excellent adhesion between sheet and core as a result of the enlargement of their surface area.
Drying of the strip edges can also be carried out before the core is compressed.
The invention also relates to an installation for carrying out the method according to the invention. An installation of this type comprises a frame provided with a guide track for guiding the core, a compression mould located on one side of the guide track and also a compression mould which is located opposite the first mould and on the other side of the guide track, at least one of the compression moulds being movable backwards and forwards in the direction of the other.
The compression moulds are so constructed that the material of the core is strongly compressed at the site of the depressions. In this way the core acquires not only the desired depressions but also the correct rigidity in its plane, which is important for further processing.
Preferably, the guide track is horizontal and the upper compression mould is stationary, whilst the lower compression mould is movable in the vertical direction.
In connection with the production of panels provided with different profiles, each compression mould preferably comprises a pressure plate to which an interchangeable moulding plate can be fitted.
The invention will be explained in more detail below with the aid of an illustrative embodiment shown in the figures.